Structure of the human glucagon class B G-protein-coupled receptor

Abstract

Binding of the glucagon peptide to the glucagon receptor (GCGR) triggers the release of glucose from the liver during fasting, thus GCGR plays an important role in glucose homeostasis. Here we report the crystal structure of the seven transmembrane (7TM) helical domain of human GCGR at 3.4 Å resolution, complemented by extensive site-specific mutagenesis, and a hybrid model of glucagon bound to GCGR to understand the molecular recognition of the receptor for its natural ligand. Beyond the shared 7TM fold, the GCGR transmembrane domain deviates from class A G protein-coupled receptors with a large ligand binding pocket and the first transmembrane helix having a “stalk” region that extends three alpha-helical turns above the plane of the membrane. The stalk orients the extracellular domain (~12 kDa) relative to the membrane to form the glucagon binding site that captures the peptide and facilitates the insertion of glucagon’s N-terminus into the 7TM domain.

Structure of the full-length glucagon class B G-protein-coupled receptor

“…3a). With this unexpected difference in conformation of the stalk region, the relative orientation of the ECD and TMD observed in the inactive GCGR-FL crystal structure differs dramatically from what was previously predicted by modeling an active conformation of GCGR-FL in complex with its endogenous ligand glucagon 7,8 (Extended Data Fig. 4a and 4b).…”

“…Four asparagine residues, N46, N59, N74 and N78 within the ECD are glycosylated by N-acetyl-D-glucosamines (NAGs). The TMD in the GCGR-FL structure features the canonical seven-transmembrane helical bundle (helices I–VII), which shares a similar conformation compared to the previously solved crystal structures of the GCGR-TMD 8,9 with Cα root-mean-square deviation (RMSD) of 1.2 Å (PDB ID: 4L6R) and 0.8 Å (PDB ID: 5EE7). The antibody mAb1 interacts with the αA helix and loops L2, L4 and L5 of GCGR-ECD as previously reported 6 .…”

“…In the previously solved structure of GCGR-TMD (PDB ID: 4L6R), the stalk region forms a 3-turn α-helical extension of helix I 8 . Surprisingly, the N-terminal portion of the stalk (G125–Q131) exhibits a different conformation in the inactive GCGR-FL structure by adopting a β-strand conformation that runs across the helical bundle flanked by ECL1 on one side and ECL2 and ECL3 on the other side (Fig.…”

“…Previous studies suggested that tertiary interactions between the ECD and TMD play a critical role in regulating receptor activity of class B GPCRs 6,7 . Structures of the ECDs of several class B GPCRs have been solved 2 , and recently, the crystal structures of the TMDs of three class B GPCRs, the human GCGR 8,9 , corticotrophin-releasing factor receptor 1 (CRF 1 R) 10 and glucagon-like peptide-1 receptor (GLP-1R) 11 , have been determined, providing insights into ligand recognition and selectivity of these physiologically important receptors. However, the structure of a full-length class B GPCR has remained elusive, thereby limiting our understanding of the molecular details accompanying peptide binding and signal transduction.…”

Cryo-EM structure of the activated GLP-1 receptor in complex with a G protein

“…More recently, the TMD structure of GCGR 8, 9 and the corticotropin-releasing factor receptor 1 10 in their inactive state provided the first insights into family B 7TMD configuration. However, our understanding of the Family B signal transduction mechanism remains limited, primarily due to the lack of structural information on active-state receptors that include both 7TMD and NTD in complex with peptide ligand.…”